1. Field of the Invention
The invention relates to a method and an apparatus for measuring the fracture toughness of a material.
2. Description of the Prior Art
In order to characterize the damage resistance of a material and understand the particular characteristics of the material which are related to damage resistance, it is necessary to identify and measure the deformation resistance of the material. Such a general measure of deformation resistance is hardness.
Recent work has been directed to measurement of the material parameter, fracture toughness, using indentation techniques and includes a model that relates fracture toughness to the applied load and the length of radial cracks that emanate from an indentation, as described in A. G. Evans and E. A. Charles, J. Am. Ceram. Soc., 59:371, 1976.
Ceramists work to develop improved, higher toughness ceramics by modifying ceramic microstructure to result in improved toughness. This work demands a fracture toughness measurement sufficiently sensitive to allow reliable correlation between microstructure and measured fracture toughness. Fracture toughness data obtained using models that relate fracture toughness to applied load and radial crack length are highly empirical in nature and differences among the fracture toughness values obtained from the different models can be large. Such inconsistencies result from the many assumptions inherent in such models. One such assumption is that the response of the material being indented is volume conserving, which neglects phenomena such as ridge formation around indents in fully dense materials and crushing damage in porous materials. The nature of the surface of the material being indented and the ratio of the surface area being indented to the indented volume is another potential source of error, since models correlating applied load and radial crack length do not take into account friction between the surface of the material being indented and the indenter that strongly affects the stress state at the tip of a crack so-produced. Hence, it is difficult to obtain consistent and reliable measurements of the fracture toughness of a material using such models.
Conventional, prior art indentation based testing procedures, such as ASTM standards for fracture toughness testing, at best, provide estimates of the fracture toughness of the material. Moreover, R-curve testing involves measurements in the long crack regime which is outside the crack size regime appropriate for study of strength controlling flaws. Finally, indentation crack length-based measurements can only be used when a well-formed crack pattern, specifically four emanating cracks is formed in the material being tested.
Thus, there exists a need for a method and apparatus for direct measurement of the fracture toughness of a material, and, particularly, for ones for obtaining that data for cracks of dimensions comparable to naturally occurring flaws in the material being characterized. It is also desirable for the method to be usable with a wide variety of crack patterns.